Combat tick, generally refers to the amount of time between attacks... Useful to know when managing DOT, quantitative ability effects, etc... For instance, a certain shadow priest ability is only triggered after 30 DOT applications, or, 30 "ticks"...
I dont know how fast basic autoattack is.
What i do know is that the DOT Set Ablaze has a 15 sec duration and does 8 ticks with the last one dealing half as much damage. I assume that that means that a DOT tick is 2 sec and that the last one ticks at second 16 for half damage.
I havnt really tried to time it. I know that some things don't take the time it is supposed to. But if that is due to lag, V&H timflow or bugs is hard to tell.